Regrowing corneas: It’s all about finding the stem cells

A restored, clear cornea grown from ABCB5-positive limbal stem cells. (Image courtesy of the researchers)Severe burns, chemical injury and certain diseases can cause blindness by clouding the eyes’ corneas and killing off a precious population of stem cells that help maintain them. In the past, doctors have tried to regrow corneal tissue by transplanting cells from limbal tissue—found at the border between the cornea and the white of the eye. But they didn’t know whether the tissue contained enough of the active ingredient: limbal stem cells.

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Results have therefore been mixed. “Limbal stem cells are very rare, and successful transplants are dependent on these rare cells,” says Bruce Ksander, PhD, of the Massachusetts Eye and Ear/Schepens Eye Research Institute. “If you have a limbal stem cell deficiency and receive a transplant that does not contain stem cells, the cornea will become opaque again.”

Limbal stem cells have been sought for over a decade. That’s where a “tracer” molecule called ABCB5—first studied in the context of cancer—comes in.

As reported in Nature last week, Ksander and collaborators at Boston Children’s Hospital, Brigham and Women’s Hospital and the VA Boston Healthcare System were able to use an antibody to ABCB5 to isolate limbal stem cells from adult human donors. Transplanted onto the eyes of mice, the limbal stem cell mixture sparked growth of clear, anatomically correct corneas—confirmed by two-photon microscopy to analyze the tissue structure.

“In every aspect, this was a normal cornea,” says Ksander. “It was avascular, had all the layers of a normal cornea, and the correct thickness, with a normal stroma underneath it.”

A bouncer for cancer cells

ABCB5+ cells are bad actors in cancer, but also turn out to be involved in normal development. (Image: leftover bacon/OpenClipArt)ABCB5 was originally discovered in the lab of Markus Frank, MD, of Boston Children’s and Natasha Frank, MD, of Boston Children’s, the VA Boston Healthcare System and Brigham and Women’s Hospital. The Franks, a husband and wife team, and co-investigators on the current study, were studying cancer stem cells, originally focusing on skin cancer. They found that ABCB5 endowed the cells with the ability to resist multiple chemotherapy agents, literally bouncing the drugs out of the cell. They later showed that melanoma growth could be inhibited in mice by injecting monoclonal antibodies against ABCB5, and that the same was true for colorectal cancer.

The Franks were also working with Ksander to study the presence of ABCB5 in two types of ocular cancer: retinoblastoma and uveal melanoma. Returning from an American Association of Cancer Research meeting in Washington, Markus Frank and Ksander found themselves on the same train. Frank changed his seat, and shared his lab’s latest data on ABCB5’s function in the skin—as well as its presence in the eye. Since the cornea is the “skin” that covers the eye, that gave the researchers an idea: They decided to test whether ABCB5 was important in the cornea’s development.

“I still have pages and pages of notes from that meeting,” recalls Ksander. “It was the shortest 6-hour train ride I’ve ever been on.”

Teaming up, and using a mouse model developed by the Frank lab, they found that ABCB5 is not only present on limbal stem cells, but is required for their maintenance and survival, preventing cell death through apoptosis. They further showed that when mice were unable to make functional ABCB5 molecules, they lost their populations of limbal stem cells and their corneas healed poorly after injury. But when the team transplanted limbal stem cells isolated from a deceased human donor, the animals’ corneas became like new.

“This finding will now make it much easier to restore the corneal surface,” says Ksander. “It’s a very good example of basic research moving quickly to a translational application.”

It’s also one of the first examples of using stem cells derived from adult human cells to construct a complete tissue.

“The mouse model allowed us for the first time to understand the role ABCB5 in normal development,” adds Natasha Frank, who is also affiliated with the Harvard Stem Cell Institute. “This should be very important to the stem cell field in general.”

Next steps

In theory, a person with a good, unaffected eye could donate his or her own limbal stem cells, with the help of an antibody to ABCB5 to home in on them. Markus Frank is working with the biopharmaceutical industry to develop a clinical-grade ABCB5 antibody.

“We can now utilize homogeneous, molecularly defined limbal stem cell populations for transplantation, a critical advantage in seeking regulatory approval of this technique,” he says.

The study also highlights the importance of cross-institution collaboration in science. “A single lab cannot do a study like this,” says Natasha Frank. “It integrates genetics, knockout mice, antibodies, transplantation—a lot of technical expertise that we were lucky came together in a very nice way.”